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McArthur Award, one of the so-called genius grants. The ammonite work
gained new credibility.
By early 1997 we had enough numbers to produce a solid papet stating
what we now know: Ammonite septa did get more complicated through
time, but that complication had nothing to do with habitation in ever
greater depth. Ironically, the eatliest of all septa designs, that used in the Pa-
leozoic Era of 500 million years ago by the earliest nautiloid cephalopods
(and a plan close to the design used today by the still-living nautilus), is by
far the best design for shell strength. But ammonites, throughout their his-
tory, had freely abandoned that design. Why? After all of this time and effort
we knew what ammonite septa did not do, but we still didn't know what they
did accomplish with their amazing complexity.
Our best guess about the real function of ammonite septa comes from
studying the still-living nautilus. The nautilus achieves nearly prefect neuttal
buoyancy in the ocean with its chambered shell. As the animal grows, it se-
cretes new chambers, and in these lie pools of liquid that must be removed. I
have long thought that the tate at which this liquid is removed is the ultimate
rate-limiting step in the growth of a nautilus, as well as in that of an ammonite.
But a new possibility has been entertained in the 1990s: Perhaps it is not the
tate at which a chambet is emptied but rather the rate at which it can be refilled.
The buoyancy system in these creatures—the extant as well as the
extinct—is a two-way street. Not only can a chambered cephalopod get
lighter in the sea (and thus reduce its buoyancy), but it can also get heavier,
by readmitting liquid into previously emptied chambers. Why do this? That
answer is simple. The Achilles heel of the shelled cephalopod design is the
fact that any removal of shell at its aperture—through shell bteakage by acci-
dental contact or, more frequently, by predatory attack—makes the cephalo-
pod suddenly mote buoyant. This is probably the most dangerous thing that
can happen to one of these creatures. When suddenly made buoyant through
shell loss, a chambered cephalopod loses its freedom of movement. Sudden
buoyancy causes it to float to the surface of the sea, where it is the helpless vic-
tim not only of creatures in the sea but of creatures in the air as well, the ubiq-
uitous birds watchfully waiting for any type of food to emerge from the depths.
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